The novel follows Callum Hunt, a young man who tries his best to fail the magical test known as the Iron Trial. If he passes, he must join the Magisterium, against his father’s wishes and his own will…

The book’s co-authors hopped onto Twitter recently for an Apple iBooks chat about how their friendship affected their writing process, and we’ve gathered the highlights below!

iBooks: Call is protagonist but starts out as an underdog. intentional?Cassandra Clare: Definitely! I’m not sure he doesn’t end as an underdog :)

iBooks: Call’s knife Miri plays an important role. How did you decide that it would be so significant?Cassandra Clare: I love a weapon with a destiny. Miri’s name has the same meaning as Call’s.

iBooks: Why did you decide to write the book in third-person?Holly Black: 3rd person allows us to have all calls thoughts and feelings, but also describe the world, which is why it’s such a popular pov for fantasy.

iBooks: When did you both decide that this story would be a series?Holly Black: We always knew it would be a series, but had to decide how many books. I convinced Cassie that five was the right number.Cassandra Clare: Five is the magic number in Magisterium! Five elements, five years, five books, five lines in the Cinquain.

iBooks: Did you guys ever have conflicting ideas about the direction of the story?Cassandra Clare: All the time but we work it out.

iBooks: What is one piece of writing advice you’ve learned from Holly?Cassandra Clare: To write the book you want to read.

iBooks: Were there any characters developed solely by one of you? Or were they all joint decisions?Holly Black: We developed everyone together. Even if one person started out with more of an idea about a character by the end they belonged to both of us. They had to or we wouldn’t both be able to get their voice right.

iBooks: Do you see yourself collaborating on more books in the future?Holly Black: We better get through these before we plan on any more. We still have four to go!

@TessMikaelson: how is writing a book together? how did you meet?Holly Black: Writing a book together is so much fun.Cassandra Clare: We met at @hollyblack ’s first signing at Books of Wonder in 2002.

‏@kristy_k13: Do you guys have any writing rituals?Holly Black: Writing rituals? Other than coffee, no. I try not too because I have to write in so many places and times.

‏@amitydelrey: what’s one thing you need to have by your side when you sit down to write?Holly Black: COFFEE.

‏@bruisedflxwer: Where do you get your writing inspiration from? XHolly Black: Inspiration comes from everywhere. From life, observing people, etc. From movies and books you love. From research.

FalsosyPalabras: How do you choose the covers for your books?Holly Black: Writers usually don’t get to pick our own covers. I know it’s surprising to hear that.

@ukuleleplay: although you don’t pick a cover, are you allowed input?Cassandra Clare: Yes, our editor David allows us a lot of input

@oicoriolanus: What’s the bigger difference of writing alone and writing with another writer?Holly Black: Someone who has to bail you out when you get stuck.

@oicoriolanus: Which character was harder to create?Cassandra Clare: Warren. We had some very anti-Warren feedback. :)

@JnnnferLawrence: What is the most number of drafts you’ve done for any of your books before it was published?Cassandra Clare: Nine!

@jaceherodale: What was your first thought about Holly?Cassandra Clare: I don’t think she knows not to go places with strangers…

@TessMikaelson: one thing you love about each other?Holly Black: Cassie can always make me see what I’m stressed about a new way & calm me down.

@goweasley: Do you cry in the end of your books?Cassandra Clare: Sometimes I cry, sometimes I LAUGH EVILLY…Holly Black: She cackles gleefully at the end of all her books.

@Cdeschaine24: Would you recommend The Iron Trial for all age groups?Cassandra Clare: Yes. It is safe for everyone even adults :)